Commit 52b6f46b authored by Hugh Dickins's avatar Hugh Dickins Committed by Linus Torvalds
Browse files

mm: /proc/sys/vm/stat_refresh to force vmstat update 



Provide /proc/sys/vm/stat_refresh to force an immediate update of
per-cpu into global vmstats: useful to avoid a sleep(2) or whatever
before checking counts when testing.  Originally added to work around a
bug which left counts stranded indefinitely on a cpu going idle (an
inaccuracy magnified when small below-batch numbers represent "huge"
amounts of memory), but I believe that bug is now fixed: nonetheless,
this is still a useful knob.

Its schedule_on_each_cpu() is probably too expensive just to fold into
reading /proc/meminfo itself: give this mode 0600 to prevent abuse.
Allow a write or a read to do the same: nothing to read, but "grep -h
Shmem /proc/sys/vm/stat_refresh /proc/meminfo" is convenient.  Oh, and
since global_page_state() itself is careful to disguise any underflow as
0, hack in an "Invalid argument" and pr_warn() if a counter is negative
after the refresh - this helped to fix a misaccounting of
NR_ISOLATED_FILE in my migration code.

But on recent kernels, I find that NR_ALLOC_BATCH and NR_PAGES_SCANNED
often go negative some of the time.  I have not yet worked out why, but
have no evidence that it's actually harmful.  Punt for the moment by
just ignoring the anomaly on those.

Signed-off-by: default avatarHugh Dickins <hughd@google.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Andres Lagar-Cavilla <andreslc@google.com>
Cc: Yang Shi <yang.shi@linaro.org>
Cc: Ning Qu <quning@gmail.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Andres Lagar-Cavilla <andreslc@google.com>
Cc: Konstantin Khlebnikov <koct9i@gmail.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 9e18eb29

Documentation/sysctl/vm.txt

+14 −0
Original line number Diff line number Diff line
@@ -57,6 +57,7 @@ Currently, these files are in /proc/sys/vm: 
- panic_on_oom

- percpu_pagelist_fraction

- stat_interval

- stat_refresh

- swappiness

- user_reserve_kbytes

- vfs_cache_pressure
@@ -755,6 +756,19 @@ is 1 second. 


==============================================================



stat_refresh



Any read or write (by root only) flushes all the per-cpu vm statistics

into their global totals, for more accurate reports when testing

e.g. cat /proc/sys/vm/stat_refresh /proc/meminfo



As a side-effect, it also checks for negative totals (elsewhere reported

as 0) and "fails" with EINVAL if any are found, with a warning in dmesg.

(At time of writing, a few stats are known sometimes to be found negative,

with no ill effects: errors and warnings on these stats are suppressed.)



==============================================================



swappiness



This control is used to define how aggressive the kernel will swap

include/linux/vmstat.h

+4 −0
Original line number Diff line number Diff line
@@ -193,6 +193,10 @@ void quiet_vmstat(void); 
void cpu_vm_stats_fold(int cpu);

void refresh_zone_stat_thresholds(void);



struct ctl_table;

int vmstat_refresh(struct ctl_table *, int write,

		   void __user *buffer, size_t *lenp, loff_t *ppos);



void drain_zonestat(struct zone *zone, struct per_cpu_pageset *);



int calculate_pressure_threshold(struct zone *zone);

kernel/sysctl.c

+7 −0
Original line number Diff line number Diff line
@@ -1521,6 +1521,13 @@ static struct ctl_table vm_table[] = { 
		.mode		= 0644,

		.proc_handler	= proc_dointvec_jiffies,

	},

	{

		.procname	= "stat_refresh",

		.data		= NULL,

		.maxlen		= 0,

		.mode		= 0600,

		.proc_handler	= vmstat_refresh,

	},

#endif

#ifdef CONFIG_MMU

	{

mm/vmstat.c

+60 −0
Original line number Diff line number Diff line
@@ -1379,6 +1379,66 @@ static DEFINE_PER_CPU(struct delayed_work, vmstat_work); 
int sysctl_stat_interval __read_mostly = HZ;

static cpumask_var_t cpu_stat_off;



#ifdef CONFIG_PROC_FS

static void refresh_vm_stats(struct work_struct *work)

{

	refresh_cpu_vm_stats(true);

}



int vmstat_refresh(struct ctl_table *table, int write,

		   void __user *buffer, size_t *lenp, loff_t *ppos)

{

	long val;

	int err;

	int i;



	/*

	 * The regular update, every sysctl_stat_interval, may come later

	 * than expected: leaving a significant amount in per_cpu buckets.

	 * This is particularly misleading when checking a quantity of HUGE

	 * pages, immediately after running a test.  /proc/sys/vm/stat_refresh,

	 * which can equally be echo'ed to or cat'ted from (by root),

	 * can be used to update the stats just before reading them.

	 *

	 * Oh, and since global_page_state() etc. are so careful to hide

	 * transiently negative values, report an error here if any of

	 * the stats is negative, so we know to go looking for imbalance.

	 */

	err = schedule_on_each_cpu(refresh_vm_stats);

	if (err)

		return err;

	for (i = 0; i < NR_VM_ZONE_STAT_ITEMS; i++) {

		val = atomic_long_read(&vm_stat[i]);

		if (val < 0) {

			switch (i) {

			case NR_ALLOC_BATCH:

			case NR_PAGES_SCANNED:

				/*

				 * These are often seen to go negative in

				 * recent kernels, but not to go permanently

				 * negative.  Whilst it would be nicer not to

				 * have exceptions, rooting them out would be

				 * another task, of rather low priority.

				 */

				break;

			default:

				pr_warn("%s: %s %ld\n",

					__func__, vmstat_text[i], val);

				err = -EINVAL;

				break;

			}

		}

	}

	if (err)

		return err;

	if (write)

		*ppos += *lenp;

	else

		*lenp = 0;

	return 0;

}

#endif /* CONFIG_PROC_FS */



static void vmstat_update(struct work_struct *w)

{

	if (refresh_cpu_vm_stats(true)) {

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